Automatic iris control means for variable focal length lens system

ABSTRACT

A servo-motor connected with potentiometers to be balanced is used for controlling the diameter of an iris of a lens system of variable focal length. One potentiometer to be used for detecting the position of control is connected with a plurality of change-over switches by way of different resistors. The change-over switches are changed in response to the change of the focal length to correct the iris with reference to the changed focal length.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an automatic iris control means for a lenssystem having a variable focal length, and more particularly to anautomatic iris control means which corrects the aperture size of theiris of a variable focal length lens system of the type that has aninterchangeable extender when the focal length thereof is changed.

2. Description of the Prior Art

When the focal length of a variable focal length lens system which as aninterchangeable extender is changed, the F-value of the lens is alsochanged. Further, the T-value of the lens system which indicates thetransmittivity of the lens system is also changed by the change of thefocal length. Therefore, the aperture size of the iris of the lenssystem must be changed when the focal length is changed. In theconventional variable focal length lens system of the type that has aninterchangeable extender, the iris is manually corrected in response tothe change of the focal length.

SUMMARY OF THE INVENTION

The primary object of the present invention is to provide a means forautomatically changing the aperture size of the iris of a variable focallength lens system in response to the change of the focal length.

The automatic iris control means in accordance with the presentinvention employs a potentiometer connected in series with resistorswhich are switched over in response to the change of the focal length ofthe lens system, said potentiometer being connected with an iris controlmeans, whereby the iris is controlled in accordance with the changedF-value and T-value in response to the change of the focal length of thelens system.

Since the iris control means in accordance with the present inventionemploys a potentiometer connected with resistors which are changed overin accordance with the change of the focal length of the lens system,the correction of the aperture size of the iris is automatically made.Further, since the iris control is automatically corrected, the lenssystem is always in a correct condition to effect a normal exposure on afilm loaded in a camera attached thereto.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a circuit view illustrated together with a block diagramshowing an embodiment of the automatic iris control means in accordancewith the present invention,

FIG. 2 is a graph which shows the relationship between the diameter ofthe iris and the voltage across the potentiometer to control the iris,and

FIG. 3 is a circuit view which shows a potentiometer employed in avariation of the present invention connected with bleeder resistancesinstead of normal fixed or variable resistors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 which shows an embodiment of the present invention,an iris control potentiometer 1 and an iris position detectingpotentiometer 2 are connected with each other by way of a series of tworesistors 3 and 4 and slidable contacts a and b slidably mounted in thepotentiometers 1 and 2, respectively. A servo-amplifier 5 is connectedwith a connecting point between the two resistors 3 and 4, and isfurther connected with a servo-motor 6. A gear device 7 is connectedwith the servo-motor 6 and the slidable contact b to transmit therotation of the servo-motor 6 to the slidable contact b. The gear device7 is further connected with an iris control means 8 so that the iris(not shown) may be controlled by the angle of rotation of theservo-motor 6. Switches 9, 9' and 10, 10' are associated with a focallength changing means in the lens system so that one pair of theswitches are changed to the other pair in response to the change of thefocal length of the lens system. In this embodiment, the switches 9 and9' are normally closed under the standard lens condition, and theswitches 10 and 10' are closed when the focal length or magnification ofthe lens system is changed from the standard value. Between the switches9, 9', 10, 10' and the potentiometer 2, resistors 11, 11', 12 and 12'are connected, respectively, so that the resistance connected with bothends of the potentiometer may be changed when the switches are changedover from one pair to the other.

Referring to FIG. 2, the voltage at the slidable contact b of thepotentiometer 2 and the diameter of the iris has a relation as indicatedby the line I changing from V1 to V2. When the switches 9 and 9' arechanged to the switches 10 and 10', the relationship between the voltageand the diameter of the iris indicated by the line I is changed to theone indicated by the line II extending from V1-ΔV to V2-ΔV.

Now it is assumed that the servo-motor 6 is balanced at a position wherethe voltage at the potentiometer 2 is Vs and the diameter of the iris isD₀ as shown in FIG. 2. In response to the change of the switches from 9,9' to 10, 10' accompanying the change of the focal length of the lenssystem, the relation between the voltage and the iris diameter ischanged from the line I to the line II. Therefore, the diameter of theiris is changed from D₀ to D₀ +ΔD to keep the balance of the servo-motor6 as shown in FIG. 2.

Therefore, by properly selecting the resistance of the resistors 11,11', 12 and 12' with respect to the change of the F-value and T-valuerequired in response to the change of the focal length of the lenssystem, the iris is automatically controlled to correct the F-value andT-value in response to the change of the focal length.

In order to accomplish the above correction simply, it is desired to usea linear control system of the iris. When the control of the irisdiameter is linear, the amount of correction of the iris diameter isalways constant irrespectively of the size of the iris diameter.

In place of the resistors 11, 11', 12 and 12', variable resistors may beemployed to facilitate the adjustment of the resistance thereof withrespect to the characteristics of the lens system.

Further, it is possible to replace the resistors 11, 11', 12 and 12'connected with the potentiometer 2 by bleeder resistors as shown in FIG.3.

When the focal length of the lens system is changed in a number of focallengths, a number of change-over switches are connected with thepotentiometer.

We claim:
 1. Means for controlling an iris of a variable focal lengthlens system comprising a servo-motor, a control potentiometer connectedwith the servo-motor for input of iris control signal, a positiondetecting potentiometer connected with the servo-motor to control theservo-motor by the balance made with reference to said controlpotentiometer, an iris control means connected with said servo-motor tocontrol the diameter of an iris of the lens system, said iris controlmeans being connected with said position detecting potentiometer so thatthe controlled position of the iris control means is detected by thedetecting potentiometer, a plurality of change-over switches connectedwith said position detecting potentiometer, said switches being changedover in response to the change of the focal length of the lens system,and resistors having different resistance connected in series with saidchange-over switches, respectively.
 2. Means for controlling an iris ofa variable focal length lens system as defined in claim 1 wherein saidresistors are bleeder resistors.
 3. Means for controlling an iris of avariable focal length lens system as defined in claim 1 wherein saidresistors are variable resistors.
 4. Means for controlling an iris of avariable focal length lens system comprising a servo-motor, a controlpotentiometer connected with the servo-motor for input of iris controlsignal, a position detecting potentiometer connected with theservo-motor to control the servo-motor by the balance made withreference to said control potentiometer, an iris control means connectedwith said servo-motor to control the diameter of an iris of the lenssystem, said iris control means being connected with said positiondetecting potentiometer so that the controlled position of the iriscontrol means is detected by the detecting potentiometer, a plurality ofchange-over switches connected with said position detectingpotentiometer, said switches being changed over in response to thechange of the focal length of the lens system, and resistors havingdifferent resistance connected in series with said change-over switches,respectively, said resistors being connected between the detectingpotentiometer and the change-over switches.